Physiology of the eye Flashcards
What comprises the lacrimal apparatus?
Lacrimal glands and lacrimal passages including puncta, canaliculi, lacrimal sac, and nasolacrimal duct
What are the three layers of the tear film from posterior to anterior?
- Mucus layer
- Aqueous layer
- Lipid or oily layer
What is the thickness of the mucus layer in the tear film?
About 0.2 mm
What does the mucus layer of the tear film consist of?
Mucin secreted by conjunctival goblet cells
What is the main composition of the aqueous layer of the tear film?
Water and small quantities of solutes such as sodium chloride, sugar, urea, and proteins
What antibacterial substances are found in the aqueous layer of tears?
- Lysozyme
- Betalysin
- Lactoferrin
What is the function of the lipid or oily layer of the tear film?
Prevents overflow of tears, retards evaporation, and lubricates the eyelids
What are the functions of the tear film?
- Keeps moist the cornea and conjunctiva
- Provides oxygen to the corneal epithelium
- Washes away debris and noxious irritants
- Prevents infection due to antibacterial substances
- Facilitates movements of the lids over the globe
What triggers reflex secretion of tears?
Sensations from the cornea and conjunctiva
What is the role of the orbicularis oculi muscle in tear elimination?
Compresses ampullae, shortens and compresses horizontal canaliculi, and creates positive pressure to force tears down the nasolacrimal duct
What is aqueous humor?
A clear, colorless, watery solution that flows from the posterior to anterior chamber
What is the normal aqueous production rate?
2.0-2.5 µl/min
What are the main functions of aqueous humor?
- Brings oxygen and nutrients to lens, cornea, and iris
- Removes metabolic products and toxic substances
- Provides an optically clear medium for vision
- Maintains intraocular pressure (IOP)
- Protects against ultraviolet-induced oxidative products
- Facilitates responses to inflammation and infection
What primary ocular structures are involved in aqueous humor formation?
- Ciliary body
- Posterior chamber
- Anterior chamber
- Angle of anterior chamber
- Aqueous outflow system
What is the conventional pathway for aqueous humor drainage?
Trabecular pathway to the Schlemm’s Canal, then into scleral and conjunctival veins, and finally into systemic circulation
What factors affect aqueous humor formation?
- Diurnal variation
- Age and sex
- Ocular inflammation
- Blood flow to ciliary body
- Sympathetic and parasympathetic system influences
What are the layers of the cornea from superficial to deep?
- Epithelium
- Bowman’s membrane
- Stroma
- Descemet’s membrane
- Endothelium
What is the primary source of nutrition for the cornea?
Aqueous humor, pre-corneal tear film, and perilimbal vessels
What are the main contents of the lens?
- Water (65%)
- Proteins (34%)
- Lipids, cholesterol, and trace elements (1%)
What is accommodation in the lens physiology?
The process by which the eye increases optical power to maintain a clear image on near objects
What causes presbyopia?
Physiological insufficiency of accommodation due to aging of the lens that results in reduced flexibility
What is the role of the retinal pigment epithelium (RPE)?
- Visual pigment regeneration
- Phagocytosis of shed photoreceptor segments
- Maintenance of the outer blood-retinal barrier
- Absorption of light
What are rods and cones responsible for in vision?
- Rods: Vision at low light levels, no color, low spatial acuity
- Cones: Active at higher light levels, capable of color vision, high spatial acuity
What is the normal size of the pupil in bright light?
3-4 mm
